I suspect there is actually a doubled 2x10 for the header and with the 2x4 on the bottom combined the the 2x4 from the top plate this is basically an I beam header. This is a very strong structural member designed to carry a lot of point load not only for the downward force but also from horizontal bend. This was likely sized according to stress loads that are at that door and is not your typical 2 2x8 you might find.

Since you will reducing the depth buy about 16 inches of the supporting member it's possible you may be going to a steel I beam with additional vertical members as well it will depend on the side forces as well.

You really should have a professional calculate what is appropriate for that support since that is one serious header.

It's going to be an expensive transom that's not very big.

Cheers.